The group represents some of the most high-profile leaders of Bahrain's Shia-led protest movement for a greater political voice in the Sunni-ruled Gulf kingdom, which is home to the U.S. Navy's 5th Fleet.

Among the eight sentenced to life is activist Abdulhadi al-Khawaja, who staged a 110-day hunger strike earlier this year in protest. The other 12 have lesser prison terms, ranging from five to 15 years, with seven of them convicted in absentia.

The decision is also likely to escalate street clashes that have taken place continuously since the Arab Spring-inspired uprising began 19 months ago. More than 50 people have been killed in Bahrain's unrest.

"We totally reject today's verdict, which is clearly not a step toward beginning to solve the issues in Bahrain," said former parliament member Abdul Jalil Khalil, a member of the country's main Shia political bloc Al Wefaq.

Claims of abuse

Shias represent about 70 per cent of Bahrain's more than 500,000 citizens, but claim they face systematic discrimination, such as being excluded from top government and security posts.

Bahrain's Sunni rulers have agreed to a series of reforms, including giving more powers to the elected parliament, but opposition leaders say the measures do not go far enough to break the ruling family's near monopoly of government control.

The anti-state convictions against the 20 opposition figures were first handed down last year by a military-led tribunal created under temporary martial law-style rules. A retrial was granted, but only limited changes in some charges were made in the hearings, said Jalil al-Aradi, one of the defence lawyers.

The 13 defendants held in Bahrain did not appear in court and family members were barred under tight security, al-Aradi said. The activists have previously claimed they suffered beatings and other abuses behind bars.

International rights groups have pressed Bahrain to free the group as a sign of outreach to opposition factions and efforts to ease criticism about the government's political crackdowns from key allies such as the U.S.

'Pattern of repression'

Al-Khawaja, who also has Danish citizenship, also has been the focus of efforts by Denmark to secure his release.

Denmark's Foreign Minister Villy Soevndal called the decision to uphold the life sentence "very disappointing" and said he would discuss possible further international action from "the very broad range of countries that in the spring supported Denmark in the demand for the release of al-Khawaja and the other human rights and democracy fighters in Bahrain."

"It is important that the international community maintains that Bahrain must respect fundamental human rights," he said. "This includes not least the freedom of speech and assembly."

Brian Dooley, director of the Human Rights Defenders Program for Washington-based Human Rights First, described the court's decision as exposing the government's reform pledges as a "hoax."

"The pattern of repression continues in the courts and on the streets, and it's hard to see how the Bahrain government intends to make any progress on human rights," he said.

The official Bahrain News Agency said the charges include "plotting to overthrow the regime" and having "foreign intelligence contacts", an apparent reference to Shia power Iran. Bahraini leaders have accused Iran of having links to the protesters. Tehran has strongly criticized crackdowns against Shias in Bahrain, but denies any active assistance.

Bahrain also faces other internal showdowns over jailed activists, including rights campaigner Nabeel Rajab, who is appealing a three-year sentence for allegedly encouraging violence.

The U.S. State Department said it was "deeply troubled" by the verdict last month, and the European Union expressed "concern" with the sentence.